A significant number of primitives are culled in graphics processing because they are back-facing, outside the view frustum or too small. Since they are not pertinent to the display, it is inefficient to process them any more than necessary.
A position only shader (POSH) has a graphics pipeline (POSH/Cull pipe) that runs ahead of the traditional render pipeline to filter out these non-visible primitives. The POSH pipeline uses a dedicated engine to operate on three-dimensional (3D) workloads to generate visibility data prior to getting them executed on a traditional render pipe. Visibility data output by the POSH pipe is used by the Vertex Fetch (VF) logic in the traditional render engine to optimize workload execution by discarding the non-visible vertices.